A van has ploughed into crowds in Barcelona's Ramblas tourist area.
Spanish police say several people have been injured in a "massive crash", while emergency services are urging people to stay away from the area around Plaça Catalunya.
Reports from the scene say people are taking cover in nearby shops and cafes.
Reuters news agency reports that emergency services have requested the closure of local metro and train stations.
El Pais newspaper said the driver of the vehicle had fled on foot after mowing down dozens of people.
Marc Esparcia, a 20-year-old student who lives in Barcelona, told the BBC: "There was a loud noise and everybody ran for cover. There were a lot people, lots of families [at the site], this is one of the most visited sites in Barcelona.
"I think several people were hit. It was horrible, there was panic. Terrible."
He said he was now taking cover inside a Starbucks coffee shop.
Details of this incident are still unclear, but vehicles have been used to ram into crowds in a series of militant attacks across Europe since July last year.
Las Ramblas
Central boulevard that runs 1.2km (0.75 miles) through the centre of Barcelona
Runs from the city's Placa de Catalunya (Catalonia Square) to the Christopher Columbus monument at the seafront.
Popular with tourists because of its market stalls, bars and restaurants
Barcelona city council restricted traffic flow because of heavy pedestrian use of the street
